1. Field of the Invention
The present invention relates to a catalyst for removing nitrogen oxides from a gas containing the same, and a method of using the catalyst. More specifically, it relates to a catalyst for catalytically decomposing nitrogen oxides, and a method of using the same. The catalyst of the present invention suffers little deterioration even in the presence of oxygen or a sulfur oxide.
2. Description of the Related Art
Nitrogen oxides in combustion waste gases discharged from, for example, industrial plants and automobiles, are the cause of photochemical smog, and the development of methods of eliminating these substances is a crucial and urgent social need from the standpoint of environmental protection. Among the nitrogen oxides, nitric monoxide (NO) is particularly difficult to remove, and various methods of removing this substance have been investigated. For example, the catalytic reduction method is considered effective, and rapid progress has been made in the development of this method. This method, however, requires the use of a reducing agent such as ammonia, hydrogen or carbon monoxide, and further, requires special equipment for the recovery or decomposition of any unreacted reducing agent. In contrast, the catalytic decomposition method does not require a special additive, such as a reducing agent, and can decompose and separate a nitrogen oxide into nitrogen and oxygen merely by passing the nitrogen oxide through a catalyst bed. Further, the process is advantageously simple, and therefore, has been regarded as the most preferable method. According to previous research, a NO decomposition activity was observed in Pt, CuO, and Co.sub.3 O.sub.4, among others, but because all of these substances are poisoned by oxygen, which is the decomposition product, they are not practically useful as catalysts.
Accordingly, the present inventors carried out intensive studies of catalysts useful for catalytic decomposition, and found that several kinds of zeolites with exchanged copper ions exhibit a stable decomposition activity. Further, the inventors found that a zeolite containing copper ions and having a specific crystal structure not only exhibits an extremely stable and high activity as a catalytic decomposition catalyst for NO, but also maintains that activity at the same level even in the presence of a sulfur oxide (see Japanese Unexamined Patent Publication (Kokai) NO. 60-125250).
The above-mentioned catalyst, however, does not have a satisfactory activity or heat resistance at high temperatures, and thus can not be put to practical use.
The present inventors have made extensive and intensive studies of the above-mentioned problems, and thus completed the present invention.